HOMOLOGOUS STRUCTURES? Vertebrate limbs have the same bone structure. Is this evidence for evolution?

The Original question was: School textbooks and lesson plans use homologous structures, such as the bones in vertebrate limbs, as evidence for evolution. How can we refute these claims?

Answer by Diane Eager

Homologous structures are organs and body parts that have the same internal organisation. For example, all vertebrates have limbs with a common arrangement of bones. This includes bird wings, bat wings, flippers, and human arms and legs. Because the functions of these limbs are so variable, yet the overall organisation is the same, they are considered evidence that all vertebrates must have evolved from a common ancestor with a similar basic arrangement of bones.

However, if you are honest, since no-one has seen a living vertebrate limb change from one kind of limb to another, similar limb structures are only evidence for descent from a common ancestor if you already believe evolution is true. The old, unpopular yet very valid argument is that such similar limb structure could just as easily be evidence for a common Designer, simply because the structure works well for the all the different functions that limbs carry out. Furthermore, no fossil links connect the different creatures that possess the same bone structure.

Now let’s take a close look at the structure and how it works for different functions.

The best way to consider how the same arrangement of bones works for different functions, is to compare your arms and legs, which have very different functions, but have the same overall arrangement of bones. Human arms need to be flexible, with a wide range of movements in order to reach things (and other parts of the body) and enable complex movements involved in manipulating objects. Human legs need to be strong and stable for weight bearing, especially as our upright stance means all the body weight goes through the legs, rather than being distributed to “a leg in each corner” like a quadruped. However, legs also need to have enough flexibility for different styles of movement (walking, running, jumping, etc.) and movement over uneven ground. The arm and the leg have the same arrangement of bones, but yet it works well for their differing function. Now consider them in more detail.

Those who know their technical anatomy will know that the upper limb includes the scapular and clavicle (shoulder blade and collar bone) and the lower limb includes the hip bones, but we will concentrate on the parts of the limbs that project out from the torso, as these are what students are commonly directed to in courses on evolution. We will also use the terms “arm” and “leg” in the general layperson’s meaning of the whole upper and lower limb respectively. (To a technical anatomist the “arm” is the part of the limb between the shoulder and elbow, the “leg” is between the knee and the ankle.)

Whatever the main need (flexibility or strength), having one bone in the upper part of a limb connected to the body with a ball and socket joint works well. The main function of the shoulder joint and humerus (upper arm bone) is to orientate the whole limb so we can reach things around us, and reach the other parts of our body. The shoulder joint has a shallow socket and a large ball to give it a large range of movement.

In the lower limb, the hip joint and femur (thigh bone) need to be strong and stable as they transmit the body weight down the limb as we stand, walk and run. Therefore, the hip joint is a ball and socket joint with a deep socket. This enables it to move in different directions, but without the extreme ranges of the shoulder.

Either way, the basic structure of one ball and socket joint attaching one large strong bone to the torso is good design, so we shouldn’t be surprised to see it repeated in the arm and leg in our own body, or in variations of the same structure in animals and birds.

Next there are two bones in parallel, attached by a type of hinge joint to the humerus or femur. This is also a useful arrangement for flexibility or strength. In the arm, the two forearm bones are named the radius and ulna. They lie in parallel to one another, but they have joints connecting them at each end to allow the bones to rotate around the long axis of the forearm. You can see and feel this movement by holding your elbow bent with your palm up. The two bones are lying in parallel to one another. Now twist your wrist so your hand is palm down. That movement occurred by the radius (the bone of the thumb side) swinging over the ulna so that it is now crossing over the ulna rather than lying side by side with it. This is an essential movement for many manipulative tasks as it further orientates the hand to where it is needed. Therefore, having two forearm bones is also a good functional design.

In the lower limb there are also two bones in parallel – the tibia (shin bone) and fibula. Between these is a tough sheet of fibrous tissue. As well as transmitting the body weight to the feet, these bones are also the attachment sites for a number of muscles that move the foot. Having two bones joined by a fibrous sheet results in a lighter structure that one large solid bony mass, provides a large surface area for muscle attachments, and allows a small amount of movement between the two bones – enough for a bit of resilience, without destabilising the weight bearing function. Again, two bones in parallel is a good functional design for this part of the limb.

In some animals and birds the equivalent bone to the fibula is small and does not extend the full length of that segment of the limb. However, it is still a useful part of the skeleton, providing extra strength, stability or muscle attachment.

Next there is a cluster of small bones. In the upper limbs these are the wrist bones, collectively named carpal bones. These give some extra flexibility to the wrist, and extend the range of movement of the hand, and help to distribute the forces passing through the wrist from all the muscle movements when using your hands.

In the lower limb, the corresponding bones, named tarsals, are larger than the wrist bones, as they continue the weight bearing function of leg. Having multiple small bones, rather than one solid mass gives the flexibility needed to walk over uneven ground.

At the end of limb we have hands with five fingers and feet with five toes. In the palm part of the hand are five bones, named metacarpals. Four of these are arranged in the slight fan shape, with one offset holding the thumb in the opposable positions. Fingers are designed for gripping and manipulation, so the bones are relatively long, with joints designed for large ranges of movement. The human hand has a relatively long opposable thumb, separated from the other fingers, but orientated to enable it to meet the tips of each of the fingers. This also facilitates being able to grip and manipulate objects.

In the foot there are also five bones, named metatarsals, but these are tightly bound together in parallel, and the toes are relatively short, with the big toe bound tightly to the other toes, sitting in parallel to them rather than opposing them. The tarsals and metatarsals are also fitted together to form an arch structure. This maintains the weight-bearing function of whole limb but enables the foot to grip the ground, or whatever surface you are moving over.

Another good reminder of how the same number of bones can be organised for different functions is to compare ape and human feet. Apes have a flat foot, rather than an arched structure, and a big toe offset from the others, like a thumb to facilitate tree climbing.

Because many vertebrate limbs end with five fingers or toes, collectively called digits, the so-called homologous limb structure arrangement is sometimes call the “pentadactyl limb”. However, in many creatures it does not end in five digits. Most birds have four toes. A horse has only one. Birds do not have five fingers on the ends of their wings either, but a bat’s wing is supported by elongated finger bones, with the equivalent of the thumb forming a claw on the top of the wing.

The standard evolutionary story is that these creatures that have fewer bones than the standard number described above is that the bones are reduced or fused, but again, this is an assumption based on an already held belief in evolution. No-one has observed this happen.

The whole concept of homologous structures comes from the way we look for common patterns and arrangements when we study the world around us. This is a useful technique to help remember and organise our knowledge. However, similar arrangements do not tell you where two similar structures came from, and they are certainly never a proof that one changed into the other. They provide no evidence for chance random evolution.

Historically, the concept homology does not originate with Darwin or evolutionary theory. It was proposed in 1843 by Richard Owen, the founder of the British Natural History Museum. However, Owen was not proposing any evolutionary theory. (He opposed Darwin’s theory when it was published later in the century.) Owen considered homology to be a common plan for all vertebrates, designed by the Creator who foreknew all its modifications.

His reason is the same as ours: similarities in structure simply indicate there is an arrangement that works, and can be varied according to the specific need. Whether it is a shared frame design in car safety, or shared bone structures, similarities actually are good evidence for creative plan and purpose by the Designer of All, the Lord Jesus Christ.

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About The Contributor

Diane Eager has a background in medicine, and was a lecturer in biomedical sciences at University of Canberra. She now works for Creation Research, writing and doing background research for our publications and DVDs, as well as giving presentations. She is particularly interested in design in living things and how living things interact with one another, as well as developing a Biblical understanding of biology.